1. Field of the Invention
The present invention relates to a device for driving an ink jet head to eject ink from the ink jet head.
2. Description of the Prior Art
A conventional ink jet head and a driver for driving the head are constructed as will be described below.
FIG. 1 shows structure of an ink jet head 1. The ink jet head 1 includes a piezoelectric ceramic plate 2, a cover plate 10, a nozzle plate 14, and a substrate 41.
The piezoelectric ceramic plate 2 is polarized in the direction indicated by arrow 5. A certain number of grooves 3 are cut into the piezoelectric ceramic plate 2 in a direction parallel to the direction of polarization. The depth of the grooves 3 becomes gradually shallower with increasing closeness to the end 15 of the piezoelectric ceramic plate 2. Shallow grooves 7 are formed adjacent to the end 15. Metal electrodes 8 are formed to the upper half of both side surfaces of each groove 3 using sputtering or some other technique. Metal electrodes 9 are formed to the side surfaces and the floor of each shallow groove 7. The metal electrodes 9 are for providing electrical connection between the metal electrodes 8 formed at either side of each groove 3.
The cover plate 10 is formed from a ceramic or resin material. An ink introduction port 16 and a manifold 18 are formed in the cover plate 10 by cutting or grinding. Using an epoxy type adhesive 20 (refer to FIG. 3), the surface of the piezoelectric ceramic plate 2 with the grooves 3 formed therein is adhered to the surface of the cover plate 10 with the manifold 18 formed therein. As a result, the tops of the grooves 3 are covered to produce ink chambers 4 (refer to FIG. 3). The ink chambers 4 are a plurality of ink channels formed at a uniform pitch across the width of the head 1. Ink fills all of the ink chambers 4a-4h, collectively denoted as ink chambers 4.
A nozzle plate 14 formed from plastic is provided with nozzles 12 in the same pitch as the pitch of the ink chambers 4. The nozzle plate 14 is adhered to the ends of the piezoelectric ceramic plate 2 and the cover plate 10 so that each nozzle 12 is aligned with an ink chamber 4.
A substrate 41 is adhered using an epoxy type adhesive to the side of the piezoelectric ceramic plate 2 opposite the side with the grooves 3 formed therein. Conductor layer patterns 42 are formed to the substrate 41 at positions corresponding to the positions of the ink chambers 4. Each conductor layer pattern 42 is connected to a corresponding metal electrode 9 at the floor of each shallow groove 7 by conductor wires 43 using well-known wire bonding techniques.
FIG. 2 shows connection of the ink jet head 1 to a driver 50 for driving the ink jet head 11 according to a print signal.
Each conductor layer pattern 42 formed to the substrate 41 is individually connected to the driver 50. Print timing is continuously supplied to the driver 50 in an ejection signal 52. Data on ink chambers 4 from which ink is to be ejected is transmitted in a print signal 53. The driver 50 applies a voltage EV to metal electrodes 8 of ink chambers 4 from which ink is to be ejected according to data transmitted in the print signal 53 at timing based on the ejection signal 52. The driver 50 applies a 0 V to metal electrodes 8 of ink chambers 4 from which ink is not to be ejected.
Next, an description of operation of the ink jet head 1 will be provided while referring to FIGS. 3 and 4.
Based on desired data, the driver 50 makes a determination that ink is to be ejected from ink chamber 4b of the ink jet head 1. The driver 50 therefore applies a positive drive voltage EV to metal electrodes 8e and 8f and a 0 V voltage to metal electrodes 8d and 8g. As a result, drive electric fields are formed in side walls 6b and 6c in directions indicated by arrows 13b and 13c respectively. It should be noted that directions indicated by arrows 13b and 13c are perpendicular to the direction of polarization 5. Side walls 6b and 6c are rapidly deformed by the piezoelectric shear effect. Therefore, the volume of the ink chamber 4b rapidly decreases and the ink pressure rapidly increases, causing an ink droplet to be ejected from the nozzle 12 that is in fluid communication with the ink chamber 4b.
In order to increase the print density of the ink jet head 1, it has been proposed in Japanese Patent Application Kokai No. HEI-4-182138 to construct a dual head with two rows of ink nozzles. The ink nozzles in each row are arranged in a staggered pattern. The piezoelectric ceramic plate of the dual head is polarized in a single direction. A plurality of grooves are cut into both surfaces of the piezoelectric ceramic plate at an equal pitch. However, grooves in one surface of the piezoelectric ceramic plate are cut shifted one half the distance of the pitch with respect to grooves cut in the other surface of the piezoelectric ceramic plate. Metal electrodes are formed to the side surfaces of the side walls that define the grooves. Because each nozzle is formed in the nozzle plate at a position corresponding to the position of a nozzle, the nozzles are also staggered. By ejecting ink droplets while transporting the ink jet head in a scanning direction, printing can be accomplished at twice the density of the above-described ink jet heat 1.
However, in the dual head described above, because the overall piezoelectric ceramic plate is polarized in one direction, the relative polarization directions of side walls defining ink chambers on opposite surfaces of the piezoelectric ceramic plate are opposite. Therefore, to eject ink in the same manner by deformation of side walls of opposite surfaces, for example, one surface must be energized with a positive voltage and the other surface must be energized with a negative voltage. In this way, both a positive and a negative power source are necessary. The drive circuit is also complicated. Both of these problems increase production costs.